Superconductivity refers to that state of metals and alloys in which the electrical resistivity is zero when the specimen is cooled to a sufficiently low temperature. The temperature at which a specimen undergoes a transition from a state of normal electrical resistivity to a state of superconductivity is known as the critical temperature ("T.sub.c ").
In the past, attaining the T.sub.c of the then known superconducting materials required the use of liquid helium and expensive cooling equipment. However, more recently superconducting materials having higher critical temperatures have been discovered. Collectively these are referred to as high temperature superconductors (HTSs). Currently, HTSs having critical temperatures in excess of the boiling point of liquid nitrogen, 77K (i.e. about -196.degree. C. or about -321.degree. F.) at atmospheric pressure, have been disclosed.
Superconducting compounds consisting of combinations of alkaline earth metals and rare earth metals such as barium and yttrium in conjunction with copper (known as "YBCO superconductors") were found to exhibit superconductivity at temperatures above 77K. See, e.g., Wu, et al., Superconductivity at 93K in a New Mixed-Phase Y--Ba--Cu--O Compound System at Ambient Pressure, 58 Phys. Rev. Lett. 908 (1987). In addition, high temperature superconducting compounds containing bismuth have been disclosed. See, e.g., Maeda, A New High-Tc Oxide Superconductor Without a Rare Earth Element, 37 J. App. Phys. L209 (1988); and Chu, et al., Superconductivity up to 114K in the Bi--Al--Ca--Br--Cu--O Compound System Without Rare Earth Elements, 60 Phys. Rev. Lett. 941 (1988). Furthermore, superconducting compounds containing thallium have been discovered to have critical temperatures ranging from 90K to 123K (some of the highest critical temperatures to date). See, e.g., Koren, et al., 54 Appl. Phys. Lett. 1920 (1989).
These HTSs have been prepared in a number of forms. The earliest forms were preparation of bulk materials, which were sufficient to determine the existence of the superconducting state and phases. More recently, HTS thin films on various substrates have been prepared which have proved to be useful for making practical superconducting devices.
Difficulty is typically encountered when trying to provide such films in three dimensional structures. For example, a tri-layer structure incorporating two layers of HTS thin film separated by an insulator or dielectric would be highly desirable. Some attempts at tri-layer structures (i.e. HTS under-layer/insulator mid-layer/HTS over-layer) have been previously described. See, e.g., W. Rauch, et al., "Planar Transmission Line Resonators From YBa.sub.2 Cu.sub.3 O.sub.7-X Thin Films And Epitaxial SIS Multilayers," 3 IEEE Trans. Appl. Supercon. 1110 (1993); S. Z. Wang, et al., "YBa.sub.2 Cu.sub.3 O.sub.7 /NdGaO.sub.3 /YBa.sub.2 Cu.sub.3 O.sub.7 Tri-layers By Modified Off-Axis Sputtering," 73 J. Appl. Phys. 7543 (1993); J. S. Horwitz, et al., "Origins of Conductive Losses At Microwave Frequencies In YBa.sub.2 Cu.sub.3 O.sub.7 /NdGaO.sub.3 /YBa.sub.2 Cu.sub.3 O.sub.7 Trilayers Deposited By Pulsed Laser Deposition," 7 J. Supercon. 965 (1994); A. E. Lee, et al., "Epitaxially Grown Sputtered LaAlO.sub.3 Films," 57 Appl. Phys. Lett. 2019 (1990); G. Brorsson, et al., "Laser-Deposited PrGaO.sub.3 Films On SrTiO.sub.3 Substrates And In YBa.sub.2 Cu.sub.3 O.sub.7 /PrGaO.sub.3 /YBa.sub.2 Cu.sub.3 0.sub.7 Trilayers," 61 Appl. Phys. Lett. 486 (1992); S. Tanaka, et al., "Epitaxial Growth of YBCO/MgO/YBCO Structure," Advances in Superconductivity III (Kajimura & Hayakawa Eds. 1991); M. Matsui, et al., "Hetero-epitaxial Growth of MgO/YBCO Thin Films by Excimer Laser Deposition," Advances in Superconductivity III, (Kajimura & Hayakawa Eds. 1991); and J. J. Kingston, et al., "Multilayer YBa.sub.2 Cu.sub.3 O.sub.7 --SrTiO.sub.3 --YBa.sub.2 Cu.sub.3 O.sub.7 Films for Insulating Crossovers," 56 Appl. Phys. Lett. 189 (1990). However, problems were found with each of these suggested structures. Such as, for example, the HTS layers were not sufficiently crystalline, the transition temperature (T.sub.c) and/or the critical current density (J.sub.c) for the HTS layers were not comparable to that for each HTS as a single layer thin film, the properties of the HTS and insulator layers degraded as the thicknesses of the layers increased, the resistivity of the dielectric layer was too high, and/or the dielectric was not pinhole-free.
Specifically, for example, in Rauch, et al., the HTS over-layer grew with increasing polycrystalline portions which resulted in degraded T.sub.c and J.sub.c properties of the over-layer; in Wang, et al., the insulating dielectric mid-layer had pinholes which resulted in microshorts between the HTS layers; in Horwitz, et al., the total tri-layer thickness was merely 0.75 .mu.m (i.e. was insufficient to provide microwave capacitive elements or other similar structures); in Lee, et al., the HTS under-layer had a low T.sub.c (between 82-87K), in Brorsson, et al., the insulating dielectric mid-layer had pinholes and the HTS over-layer was polycrystalline; in Tanaka, et al., no electrical data for HTS layers was reported; in Matsui, et al., no electrical data for HTS layers was reported and the HTS/insulator interfaces were disrupted; and in Kingston, et al., the total tri-layer thickness was merely 1 .mu.m (i.e. was insufficient to provide microwave capacitive elements or other similar structures).
Some of these problems may be due to the HTS growth mode (i.e. two or three dimensional), due to an oxygen deficiency in an HTS layer, and/or due to differences in thermal expansion coefficients of the substrate and the tri-layer materials.
It should be noted that the present specification uses the terms "two-dimensional growth mode" and "three-dimensional growth mode" in a qualitative manner, and, as so used, these terms may be used in a different sense by other workers. With respect to the present specification, growth of a material as a thin film can occur in a two-dimensional mode or in a three-dimensional mode depending on the growth conditions. In two-dimensional modes growth of the film takes place in a layered manner, resulting in smooth surfaces and high crystal quality. In three-dimensional modes growth takes place unevenly and results in defective and undesirably rough surfaces. In addition, the roughness and defectiveness of layers grown in three-dimensional modes increases with increasing thickness thereby limiting the ability to grow quality layers of sufficient practical thickness.
For example, using a pulsed laser deposition (PLD) process NdAlO.sub.3 grows in a two-dimensional mode when the substrate temperature ("T.sub.B ") is 800.degree. C. and the oxygen pressure ("P(O.sub.2)") is 20 mTorr; and in a three-dimensional mode when the T.sub.B is 800.degree. C. and P(O.sub.2) is 500 mTorr. If NdAlO.sub.3 were used as the insulator mid-layer in a tri-layer structure, the NdAlO.sub.3 mid-layer would be grown over an HTS under-layer and an HTS over-layer would be grown over the NdAlO.sub.3 mid-layer. However, the oxygen pressure used to allow two-dimensional growth of NdAlO.sub.3 is insufficient to maintain thermodynamic stability of an HTS such as YBCO. The growth of a NdAlO.sub.3 mid-layer on a YBCO under-layer under two-dimensional growth conditions would result in a progressive loss of oxygen from the YBCO under-layer crystal structure to a point at which the YBCO would decompose into various oxides.
Continuing with the same example, growing the NdAlO.sub.3 mid-layer under three-dimensional growth conditions is unacceptable because the resulting NdAlO.sub.3 mid-layer has a rough surface. An insulator mid-layer with a rough surface will adversely affect the ability to grow an acceptable HTS over-layer. For example, growing a YBCO layer on a rough surface insulator layer, causes the YBCO to grow in a three-dimensional mode due to the rough template provided by the rough surface. In addition, a YBCO layer grown on a rough surface tends to be polycrystalline and tends to have degraded crystalline properties.
Therefore, a method is needed in which an insulator mid-layer can be grown in a two-dimensional mode on an HTS under-layer without substantially permanently adversely affecting the HTS under-layer.
As is mentioned above, some of the problems of prior tri-layer structures may be due to an oxygen deficiency in an HTS layer, particularly in the HTS under-layer. Typically, in single layer YBCO films, oxygen is introduced after growth of the film by cooling the film in an oxygen atmosphere where P(O.sub.2) is 760 Torr. The additional oxygen increases the T.sub.c of the film. Because oxygen diffuses rapidly into YBCO, a fully oxygenated YBCO film may be obtained even upon rapid cool down where the oxygen pressure is raised to about 760 Torr near the growth temperature. However, in a tri-layer structure an HTS over-layer and an insulator/dielectric mid-layer act as barriers to oxygen diffusion into an HTS under-layer. The typical cool-down conditions for single layer films are insufficient when used for tri-layers as the YBCO under-layer is oxygen deficient and non-superconducting after such processing.
Elaborate and expensive processes have been used to oxygenate YBCO layers when standard heat treatments were unsuccessful due, for example, to the YBCO layer being an under-layer and the structure being thick. For example, Ockenfu.beta., et al., used a radio frequency oxygen plasma to oxygenate NdGaO.sub.3 /YBCO bilayers when they were unable to oxygenate the bilayers by standard heat treatments. See, G. Ockenfu.beta., et al., "In-Situ Low Pressure Oxygen Annealing of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. Single- and Multilayer Systems," 243 Physica C 24 (1995). In another example, Chew, et al., used a microwave oxygen plasma to oxygenate structures which were more than 1 .mu.m thick when they were unable to oxygenate the structures by standard heat treatments. N. G. Chew, et al., "Importance of Process Control for Superconductor Thin Film Growth," S3-1 ISTEC Workshop on Superconductivity 97 (1995). Therefore, a technique is needed to provide a fully oxygenated YBCO under-layer in a tri-layer structure without requiring to elaborate or expensive plasma treatments.
As is mentioned above, some of the problems of prior tri-layer structures may be due to differences in thermal expansion coefficients of the substrate and the various tri-layer materials. Such differences in thermal expansion coefficients can typically result in cracking in the layers, particularly as the thicknesses of the layers is increased. Therefore, a method is needed in which thick layers are used in a tri-layer structure and cracking of the layers is prevented.
Also, conventional HTS capacitors for microwave applications are typically made using HTS layers grown on both sides of a planar substrate (which serves as the dielectric). Such substrates have thicknesses on the order of several hundreds of microns. However, if the thickness of the dielectric could be made smaller, then the size of a capacitor for a given capacitance would correspondingly decrease, and/or the capacitance for a given size of capacitor would correspondingly increase. A method is needed to form HTS capacitors with smaller dielectrics so resulting capacitors could be smaller and/or stronger.
Prior to now there has been no completely satisfactory way to adequately prepare a tri-layer structure in which an insulator/dielectric mid-layer can be grown in a two-dimensional mode on an HTS under-layer without adversely affecting the HTS under-layer in a way which is somewhat easily reversible. In addition, prior to now there has been no way to provide a fully oxygenated HTS under-layer (e.g. YBCO) in a tri-layer structure in a somewhat easy and inexpensive manner (i.e. using standard heat treatments, e.g., without requiring use of elaborate or expensive plasma treatments). Furthermore, prior to now there has been no way to prepare a tri-layer structure in which thick layers are used and cracking of the layers is avoided. Also, prior to now there has been no way to provide smaller HTS capacitors with increased capacitances.